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THE 



3 OF THE INVISIBLE HEAVENS 



BY 



Sir ROBERT BALL 




NEW YORK. 

STREET & SMITH, Publishers, 
238 William Street. 




; THE LIBRARY OF 
CONGRESS, 

j Two Copies Recc - 

! MAR 27 1903 j 

I Copyright kntry 
| CLASS CL XXc. 

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Copyright, 1903 
By Sir ROBERT BALL 

The Scale of the Invisible Heavens 



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THE SCALE OF THE INVISIBLE HEAVENS. 



It is possible to demonstrate that objects do certainly 
exist which are not only utterly screened from view so 
far as our present resources extend, but which there is not 
the least reason to anticipate that any future discoveries 
can introduce to our ken, The present state of science 
indeed forces us to believe that there is around us an 
invisible universe which immeasurably exceeds the uni- 
verse we can see. This is indeed one of the most striking 
conceptions which Nature has to offer to our contem- 
plation. There are different ways in which it can be 
presented to us, and I shall try to develop it with such 
detail as its importance deserves. 

The sun, to which we owe so much, is no doubt a 
potent agent of illumination, within the narrow limits, the 
relatively narrow limits I ought rather to say, of our 
solar system. But for purposes of illumination through 
the length and breadth of the universe, the sun is as utterly 
inadequate as a farthing rushlight would be for the il- 
lumination of a continent. We are apt quite naturally to 
attribute to the sun the possession of a peerless splendor. 
We must, however, remember that it is only because the 
earth is so close to the sun that it receives so abundantly 
of its radiation. How slender must be the solar effect in 
illuminating or warming the universe generally may be 
inferred from the well-known fact that many of the 
bright stars, for example Sirius or Arcturus, are in- 
trinsically far more brilliant than the sun, yet how feeble 
is the twinkle which they transmit to our point of view ! 
Any objects which lie in the immediate vicinity of Sirius 
or Arcturus may no doubt derive from either of those 
bodies an illumination quite as splendid as, or even far 
more splendid than, that which is supplied to the earth 



4 The Scale of the Invisible Heavens. 

by the sun. But sun and stars alike are equally ineffective 
as illuminating agents when the length and breadth of the 
starry spaces are considered. 

The question as to whether an object shall be visible to 
us or not is a one of illumination. If the object be bright 
enough, and if the distance at which it is situated be 
not too great for the degree of brightness which the 
object possesses, then that object will generally be visible. 
We must, however, provide that the sensibility of the 
retina to the impression of light is not reduced by the 
presence of an undue quantity of diffused light from some 
other source. A star is generally just visible to us at 
night by the unaided eye if' it possesses that degree of 
brightness indicated in the language of the astronomer 
when he says that the star is of the sixth magnitude. 
If that star were moved further away then it would 
presently cease to be visible to the unaided eye, though 
it might still be discerned with the aid of a telescope. 
The larger the telescope the greater the depth to which 
it is able to probe into space. Indeed it may be said 
that a star just visible to the unaided eye would have 
to be removed to a distance about one thousand times 
greater before it had ceased to be a visible point in the 
great American telescopes at Lick or Yerkes, or in the 
great reflector of Lord Rosse at Parsonstown. Were the 
star to be translated ten thousand times as far as when 
just visible to the unaided eye it would apparently be 
then utterly beyond the reach of any telescope at present 
existing. It seems, however, possible that even this dis- 
tance might not be so great as to preclude some stars from 
recording their impressions in a photographic apparatus 
when an extremely long exposure had been given. 

Though stars abound overhead in daylight, yet we can- 
not in general see these stars. The reason is simply that 
the nerves of the retina are so strongly acted upon by the 
abundant floods of daylight, that the twinkle of even the 
brightest star fails to produce any recognizable impres- 
sion. No doubt stars, or at all events the brighter stars, 
can be rendered visible in daylight with our telescopes. 
Supposing, however, that we have lived in perpetual day- 
light, as we might have done if it had happened that 
the earth turned round the sun, with the same face 



The Scale of the Invisible Heavens. 5 

always directed thereto, just in the same way as the 
moon goes round the earth, then if we had no telescopes 
we should never have become aware of the existence of 
the stars. Under very exceptional circumstances we 
might, indeed, have occasionally glimpsed the planet 
Venus, but with this possible exception we should never 
have known anything about any other bodies in the uni- 
verse, save the sun and the moon. All that glorious 
sidereal spectacle which is disclosed to our gaze at night 
would have been utterly unknown. The starry firmament 
would have formed an invisible universe. 

Suppose under these circumstances that the sun was to 
be suddenly eclipsed, then the whole of that universe, 
previously invisible and unknown, would have been in- 
stantly displayed to the astonished observer. There he 
would behold for the first time the Great Bear, Orion and 
other glorious constellations, while stretching across the 
sky he would see the marvelous yet delicate glow of the 
Milky Way. If the observer were further told that every 
single unit in this display of twinkling points of light in- 
dicated the existence of a sun, in many cases quite as 
great and as glorious as that sun which was the familiar 
object in his skies ; if he were led to realize that these suns 
existed in scores of millions, and that many of them may 
be surrounded by systems of planets, attending upon 
them just in the same way as the planets revolve around 
the sun; then indeed he would see that the universe as 
known to him before the eclipse was as nothing compared 
with that hitherto unseen universe of which he had for 
the first time been permitted to obtain a brief view. The 
problem of the invisible universe would be one which 
would astound his imagination. 

On a dark night the mariner can see no more of a 
distant coast than the beacons maintained for his guid- 
ance. For every lighthouse which may be counted around 
the coasts of Great Britain there are within the circuit of 
these coasts thousands of fields, uncounted myriads of 
trees, there are many lakes and rivers, there are villages, 
towns, cities, and great numbers of inhabitants. So, too, 
for every one of the visible stars which can be counted 
in the skies there must be hundreds or thousands, indeed 
there must be millions of other objects utterly beyond our 



6 The Scale of the Invisible Heavens. 

ken. Reflect on the number of luminous stars which the 
heavens contain, think of the thousands of stars which 
are visible to the unaided eye, think of the tens of thou- 
sands of stars which are visible in small telescopes, think 
of the hundreds of thousands of stars which are visible 
in a moderate telescope, and the abounding millions of 
stars which are disclosed by our mightiest instruments, 
and which are represented on our most sensitive photo- 
graphic plate. Then remember that each one of these 
stars is as it were a luminous beacon, and that the in- 
visible objects must, according to all probabilities, be 
vastly more numerous than the beacons themselves. In 
this way we begin to realize that for each body which 
we see glowing as a fervent star there must be thousands 
or millions of other bodies often as large, often doubtless 
a great deal larger, than the luminous stars. We do not 
see the great majority of celestial objects from the simple 
fact that they do not generally possess temperatures suf- 
ficient to make them glow in the manner necessary for 
visibility. If indeed the mind is baffled in the attempt to 
comprehend the scale of the universe which contains, as 
we know it does contain, millions of stars, many of them 
as bright and as glorious as the sun, what are we to think 
when it is represented to us that each one of these stars 
is itself only one, among millions of objects, which hap- 
pens to be rendered visible by the fortuitous circumstance 
of temperature? 

We may illustrate the line of reasoning that we have 
followed in another way. Some of us have seen those 
beautiful fire-flies which in clouds of dancing points of 
light form a striking feature after the night has fallen 
in certain warm latitudes. Suppose that some celestial 
being who was taking a survey of our earth at night 
when all artificial sources of illumination were absent, 
was trying to obtain some notion as to the nature of the 
animated inhabitants of the earth. His survey being made 
in the darkness would necessarily preclude him from 
being able to perceive the greater number of living forms. 
The huge bulk of the elephant or of the rhinoceros must 
pass unnoticed; the stately giraffe would not be visible; 
lions, tigers, and bears would be as invisible as cows or 
sheep. Birds of every size and of every hue must be 



The Scale of the Invisible Heavens. 7 

utterly unknown to an observer so circumstanced, and 
innumerable hosts of minor creatures would remain un- 
detected. Suppose, however, the observer looked atten- 
tively through the darkness he might discern here and 
there a little gleam of light from a glow-worm on a mossy 
bank; he might detect phosphorescent sparks in the sea- 
water; here and there he would be gratified by the sight 
of a cloud of fire-flies dancing about in the darkness. If 
this celestial being, having duly noticed these things, 
having counted the number of glow-worm twinkles that 
he could find, and having depicted or measured the phos- 
phorescent points and the clusters of fire-flies, were 
straightway to depart and say that now he knew all about 
the distribution of life on this earth, how absurdly would 
he have been mistaken ! No doubt it may be admitted that 
he w T ould have seen a very large number of creatures. 
The number of fire-flies in their clustering millions may 
really rival, for aught we can tell, the number of stars in 
the Milky Way. But how ludicrously incomplete would 
be the knowledge of the natural history of this earth 
which could possibly be obtained by one whose only op- 
portunity for observing the animal life on our globe was 
obtained under the limitations we have sketched ! All the 
more important forms of life would be quite unknown 
to such an observer, he would really have perceived only 
an infinitesimal part of the total life on the globe. Those 
creatures alone would be visible to him, which possessed 
intrinsic luminosity. The creatures so endowed form, 
it may be, an interesting, but certainly only a most insig- 
nificant, part of animated nature. 

In like manner when we raise our eyes to the skies we 
see, it is true, a myriad of glittering gems, but these are 
only the glow-worms and the fire-flies of the universe. 
That is to say, they are the objects which are visible in 
virtue of the light which they themselves dispense, while 
objects that are not endowed with the capacity for radiat- 
ing luminosity must be as invisible to us as the birds and 
beasts on the earth would be to the spectator whom we 
have just been considering. There can, however, be 
little reason for doubting that the invisible objects in 
the universe exceed in number those which are visible in 
consequence of their luminosity in a ratio quite as re- 



8 The Scale of the Invisible Heavens. 

markable as that in which the ordinary animals devoid 
of luminosity exceed those which possess phosphorescent 
qualities. 

A star is a mass of matter heated to such an extent 
that its effulgence is perceived far and wide. It must, 
however, be borne in mind that for a portion of matter 
to be heated so highly is always a more or less excep- 
tional phenomenon. From the very nature of the case, 
the condition it implies can be only a temporary one. We 
find little difficulty in conceiving even an eternal existence 
of matter at a temperature no greater than that of the 
surrounding space; but when matter is heated to incan- 
descence it is in the very nature of things that this condi- 
tion must be transient. The high temperature may last, 
no doubt, as the high temperature of the sun has lasted, 
for millions of years. It cannot, however, be perpetual, 
so that when at last that portion of matter cools down 
again to the temperature of space, there it may remain 
to all eternity, unless in so far as by the chapter of ac- 
cidents it -may be again kindled into a resumption of 
temporary luminosity. The normal and ordinary state 
of the matter in the universe is to be cold, non-luminous, 
and therefore utterly invisible to us. Those portions of 
matter which are at any moment luminous must certainly 
be very greatly inferior in numbers to those which are at 
the same time in the normal condition. Every line of 
reasoning demonstrates that the material universe so far 
as it is visible can only be an almost inconceivably small 
fragment of that unseen universe which, from not pos- 
sessing the necessary quality of luminosity, is effectually 
shrouded from view. 

But the invisible objects are not stars alone, invisible 
nebulae of astonishing dimensions are sometimes repre- 
sented on our sensitive photographic plates. It remains 
for us now to give a brief account of one remarkable in- 
stance known of an invisible nebula, which is nevertheless 
now very well known by its photographs, 

I do not suppose that any constellation in the heavens 
has been the object of greater attention than the famous 
group known as the Pleiades. In the first place, the ar- 
rangement of the cluster of stars lying so closely together 
is so remarkable that, from the very earliest times, it must 



The Scale of the Invisible Heavens. 9 

have arrested attention. Half a dozen stars are readily 
discerned by most eyes, and those gifted with excep- 
tionally acute vision are able to see nearly double that 
number. The fact that the Pleiades are apparently so 
crowded together no doubt makes the discrimination of 
the fainter points more difficult than would be stars of the 
same brightness but more widely separated. 

With the slightest telescopic pow r er the numbers of the 
Pleiades are found to be enormously increased. Sixty- 
nine stars in the group have been catalogued and have 
had numbers assigned to them by Dr. Elkin, and the 
places of the stars so selected have been determined with 
the utmost care. 

One of the objects which Dr. Elkin had in view when 
he undertook so considerable a labor was to provide 
astronomers with a convenient scale for the measurement 
of distances on the celestial sphere. We can now test 
the accuracy of our micrometers by comparing the results 
they give when applied to the stars in the Pleiades with 
the measurements given by Dr. Elkin. 

It must not be supposed that the list of stars just re- 
ferred to includes all the stars in the group. Their num- 
ber is indeed vastly greater. With every increase in the 
power of the telescope more and more stars are brought 
within the range of vision, while what the most powerful 
telescope is able to do in this direction is vastly trans- 
cended by the results which we obtain when we take a 
photograph with a long exposure. The brothers Henry 
in Paris have obtained more than two thousand stars on 
a single plate directed to the Pleiades. Nor is there the 
least reason to think that the full tale of stars in the group 
has been even yet ascertained. Each increase in the sen- 
sibility of the plate, or in the duration of its exposure 
invariably brings with it an increased number of the stars 
which are represented. 

The distance by which we are separated from the 
Pleiades is brought forcibly before us when we try to 
discover the movements of its stars. The measure- 
ments that Dr. Elkin has taken have been carefully com- 
pared with those obtained fifty years previously by Bessel. 
From a study of the results it is possible to determine 
how far the stars have shifted their apparent relative posi- 



io The Scale of the Invisible Heavens. 

tions in the lapse of half a century. The result is ex- 
tremely remarkable. It is plain that the Pleiades have 
really undergone no changes of any appreciable amount 
in their relative positions. Indeed, we may say that this 
system is characterized by the absence of apparent motion 
of the stars to or from each other. The result of this is 
in the highest degree instructive. It is certain that the 
stars in the Pleiades must be acted upon by their mutual 
attractions. Instead of the system being at rest, as the 
observations would seem to indicate, it is certain that the 
stars must one and all be animated by incessant move- 
ments. There is only one way by which we can reconcile 
the apparent discrepancy between this reasoning and the 
apparent facts of observation. The movements must cer- 
tainly exist; if therefore they are imperceptible, the only 
possible explanation must be that the distance of the 
Pleiades from our solar system is so great that the ap- 
parent movements of the stars during the time they have 
been submitted to our examination are not considerable 
enough to be appreciable when viewed from this distance. 
Yet it cannot be doubted that those motions are really of 
enormous intrinsic amount. It is certain that each of the 
stars in the Pleiades has moved many millions or, more 
likely, thousands of millions of miles since Bessel ob- 
served it fifty years ago. If such a star still remains in 
the same apparent place, the explanation can only be that 
its distance from our earth is so stupendous that all those 
millions of miles are as nothing in comparison. The 
consequences to which we are thus led are confirmed in 
many other ways. They demonstrate that the Pleiades 
must certainly be several millions of times as remote from 
the earth as is the sun itself. 

An acute" astronomer, Tempel, studying the Pleiades 
in the pure skies of Venice in the year 1859, discovered 
faint traces of a nebula or haze of glowing gas appended 
to one of the stars. The observation was, however, such 
a difficult one that it was but rarely repeated, and many 
of those who searched for this nebula, even with power- 
ful instruments, could observe no trace of it whatever. 
From time to time, however, there were occasional con- 
firmations of its existence. But the available informa- 
tion about the nebula in the Pleiades was extremely 



/ 



The Scale of the Invisible Heavens. n 

scanty, until the appearance in recent years of a new ob- 
server with an eye of extraordinary delicacy, and pos- 
sessed of a unique faculty for recording his impressions 
with absolute accuracy. This new observer is the dry 
plate of the photographer. The chemicals in the plate, 
as the Rontjen Rays have demonstrated in an extraordi- 
nary manner, possess the power of seeing things that are 
wholly invisible to our ordinary vision. It is not merely 
that the photographic plate can see more than is re- 
vealed to the unaided eye. A photograph will often show 
more than the most powerful telescope displays on the 
very finest night to the most trained observer. It is to 
the photographic plate that we are indebted for our 
knowledge of the extent and magnificence of the nebula 
in the Pleiades. 

Here we should specially note the extraordinary pe- 
culiarity of the sensitiveness which the photographic plate 
possesses. It is not merely that the plate catches and ex- 
hibits rays of light which are too faint to produce impres- 
sion on the ordinary nerves of vision. In such cases it is 
rather a question of the quality of the light, than of its 
quantity which is concerned. There are certain kinds of 
light that, no matter how great be the amount in which 
they are present, will impress the organs of vision little or 
not at all. These same kinds of light will, however, in 
some cases act most energetically on the substances con- 
tained in the photographic plate. It is clear that the ma- 
terial, whatever it may be, which forms the nebula in the 
Pleiades, has a radiance of this invisible character. The 
light that it pours forth is mainly that kind of light which 
appeals not to vision, but which appeals to the peculiar 
sensibility of the photographic plate. It is indeed very 
doubtful how far the faint wisp of nebula which Tempel 
discovered in the Pleiades can be identified as a part of 
that copious nebula whose existence is so clearly mani- 
fested on the photographs. 

Great as has been the interest justly attached to the 
Pleiades from all time ; interesting as are the innumerable 
legends which are associated with the group; remarkable 
as are the labors of the painstaking astronomers who have 
devoted themselves to the minute study and measurement 
of the stars which the Pleiades contain, it must yet be ad- 



12 The Scale of the Invisible Heavens. 

mitted that it has been reserved to these latter days to dis- 
cover the most remarkable circumstance connected with 
this little consellation. The fact that the Pleiades are 
surrounded by this marvelous invisible nebula will invest 
the wonderful group with still greater interest for the 
future. We have now to regard the constellation as con- 
sisting not merely of the ten or a dozen stars that can 
be seen with the unaided eye, not merely of the hundreds 
of stars that the telescope shows, and the thousands of 
stars that are on the photographic plate; we have to 
speak of the whole group as bathed and immersed in a 
marvelous yet invisible fire-cloud. Must it not have been 
with some poetic pre-vision of these discoveries that Ten- 
nyson wrote: 

"Many a night I saw the Pleiads 

Shining through the mellow shade, 
Glitter like a swarm of fire-flies 
Tangled in a silver braid?" 



I 




ATLANTIC SERIES No. 43 



/A A A A A A A A A A A A 



THE 



Scale of the Invisible Heavens 

/ 



BY 



Sir ROBERT BALL 



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STREET & SMITH, PUBLISHERS, NEW yORK 



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